Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
U.S. Pat. No. 6,085,528 (the “PLNG Patent”) entitled “Improved System for Processing, Storing, and Transporting Liquefied Natural Gas”, describes containers and transportation vessels for storage and marine transportation of pressurized liquefied natural gas (PLNG) at a pressure in the broad range of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a temperature in the broad range of about −123° C. (−190° F.) to about −62° C. (−80° F.). Containers described in the PLNG Patent are constructed from ultra-high strength, low alloy steels containing less than 9 wt % nickel and having tensile strengths greater than 830 MPa (120 ksi) and DBTTs (a measure of toughness, as defined in the Glossary) lower than about −73° C. (−100° F.). As discussed in the PLNG Patent, at the preferred operating pressures and temperatures of the invention described therein, about 3½ wt % nickel steel can be used in the coldest operating areas of a PLNG plant for the process piping and facilities, whereas more expensive 9 wt % nickel steel or aluminum is generally required for the same equipment in a conventional LNG plant (i.e., a plant for producing LNG at atmospheric pressure and about −162° C. (−260° F.)). Preferably, high strength, low alloy steels with adequate strength and fracture toughness at the operating conditions of the PLNG plant, are used to construct the piping and associated components (e.g., flanges, valves, and fittings), pressure vessels, and other equipment of the PLNG plant in order to provide economic advantage over a conventional LNG plant. U.S. Pat. No. 6,212,891 the (“Process Component Patent”) entitled “Process Components, Containers, and Pipes Suitable For Containing and Transporting Cryogenic Temperature Fluids”, describes process components, containers, and pipes suitable for containing and transporting cryogenic temperature fluids. More particularly, the Process Component Patent describes process components, containers, and pipes that are constructed from ultra-high strength, low alloy steels containing less than 9 wt % nickel and having tensile strengths greater than 830 MPa (120 ksi) and DBTTs lower than about −73° C. (−100° F.). U.S. Pat. No. 6,460,721 (the “Non-load-bearing Liner Container Patent”), entitled “Systems And Methods For Producing And Storing Pressurized Liquefied Natural Gas”, describes containers and transportation vessels for storage and marine transportation of pressurized liquefied natural gas (PLNG) at a pressure in the broad range of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a temperature in the broad range of about −123° C. (−190° F.) to about −62° C. (−80° F.). Containers described in the Non-load-bearing Liner Container Patent are constructed from (a) a load-bearing vessel made from a composite material, said vessel being suitable for withstanding pressures of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and temperatures of about −123° C. (−190° F.) to about −62° C. (−80° F.); and (b) a substantially non-load-bearing liner in contact with said vessel, said liner providing a substantially impermeable barrier to said pressurized liquefied natural gas. The PLNG Patent, the Process Component Patent, and the Non-load bearing liner Container Patent are hereby incorporated herein by reference.
The PLNG Patent and the Process Component Patent utilize ultra-high strength, low alloy steels as the connecting theme between the PLNG plant and the containers used for storing and transporting the PLNG. If use of the steels for constructing the containers did not provide a commercially viable means for storing and transporting the PLNG on marine vessels, then any use of the steels in the plant would be meaningless since there would be no mechanism for commercially transporting the PLNG produced by the plant. Conversely, while use of the steels in the PLNG plant generates some economic savings over conventional LNG operations, the most substantial economic benefit is derived from the enormous simplification (and consequent cost reductions) in the plant. Because of its relatively simple design, the PLNG plant is substantially cheaper than a conventional LNG plant of similar capacity. Additionally, while use of the steels in the PLNG transportation system is commercially viable and does generate some economic savings over conventional LNG operations, the weight of the steel containers is high compared to that of its PLNG cargo, resulting in a relatively low cargo-carrying capacity performance factor (PF). The PF for compressed fluid storage containers relates the pressure exerted by the cargo (P) to the volume (V) of the container and the weight (W) of the container by the equation PF=PV/W. What is currently missing from the all-steel PLNG system (i.e., plant plus transportation) is a combination of the PLNG plant with a low cost, higher PF, container-based transportation system that is capable of handling PLNG.
High-performance fibers, which offer high strength-to-weight ratios, are used to construct lightweight composite-overwrapped pressure vessels. Such lightweight pressure vessels have been used extensively in the aerospace industry and for life-support systems such as emergency breathing apparatus for professional firefighters, miners, and rescue workers. These pressure vessels are also used for portable oxygen for medical applications and for flight crew and passengers. Seal et al. (U.S. Pat. No. 5,822,838) describe the two primary technologies used in the design of such high-pressure gas containment systems. The first approach, the most prevalent, uses thin metallic liners (e.g. aluminum) that yield during the service cycle because each pressure cycle results in fiber/composite strain higher than the yield strain (or elastic capability) of the liner. This generally limits the cycle life of the liner and hence of the pressure vessel. In this approach, the liner is non-load bearing; it provides essentially no contribution to carrying the structural load, but only serves as a gas-permeation barrier for the pressure vessel. Such liners are typically bonded to the composite. In the second approach, a material with a higher elastic range relative to the fiber strain during the pressure service is selected for the liner. This increases the liner life since the liner remains elastic during the operating pressure cycles. The liner is also required to share the structural load and is therefore characterized as load-bearing. Typically, the composite is applied only in the hoop direction since the liner must be thick enough to operate in the elastic range. Seal et al. prefer a titanium liner. Both U.S. Pat. No. 5,577,630 (Blair et al.) and U.S. Pat. No. 5,798,156 (Mitlitsky et al.) describe lined, composite pressure vessels for storing and transporting compressed natural gas.
Use of such composite-overwrapped pressure vessels in cryogenic service introduces another problem inherent in the design due to the difference in the CTE, or coefficient of thermal expansion or contraction, of the liner material and the composite. Typical values of CTE are about −5.6×10−7 m/m/K (−1×10−6 in/in/° F.) for carbon fiber composite, about 3.3×10−6 m/m/K (6×10−6 in/in/° F.) for glass fiber composite, and about 7.2×10−6 m/m/K (13×10−6 in/in/° F.) for aluminum. As a typical composite pressure vessel is cooled to cryogenic temperatures, the liner, which is typically aluminum, tends to contract more than the composite material causing the liner to separate from the windings and subsequently causing pre-mature failure. Innovative approaches to address the CTE problem are the subject of several patents, e.g., U.S. Pat. No. 4,835,975 (Windecker), U.S. Pat. No. 3,830,180 (Bolton), and U.S. Pat. No. 4,073,400 (Brook et al). For example, Windecker (U.S. Pat. No. 4,835,975) proposes using a low-carbon steel liner (having a CTE of about 3.1×10−6 m/m/K (5.5×10−6 in/in/° F.)) and fiberglass composite which have comparable CTE's to avert the problem.
U.S. Pat. No. 3,830,180 (“Bolton”) discusses use of a double-walled, composite cylindrical vessel configuration for transport of regular LNG, i.e., LNG at atmospheric pressure and at temperatures of about −162° C. (−260° F.). However, the load-bearing, inner wall of Bolton's vessel is designed for a maximum pressure of approximately 0.34 to 0.41 MPa (50 to 60 psi) and, thus, Bolton's vessel is not suitable for transport and storage of PLNG. Further, Bolton does not discuss liner material but proposes the use of a plastic material, such as FRP pipe (fiber reinforced plastic pipe), or other suitable material “capable of enduring exposure and stress at cryogenic temperatures” for construction of the inner and outer walls of the vessel; however use of FRP necessitates use of a liner since the resin for the FRP will micro-crack at cryogenic temperatures and will not be impermeable to the product, as will be familiar to those skilled in the art.
S. G. Ladkany, in “Composite Aluminum-Fiberglass Epoxy Pressure Vessels for Transportation of LNG at Intermediate Temperature”, published in Advances in Cryogenic Engineering, Materials, volume 28 (Proceedings of the 4th International Cryogenic Materials Conference), San Diego, Calif., USA, Aug. 10, 1981–Aug. 14, 1981, discusses the design of pressure vessels for the transportation of liquefied natural gas (LNG) at temperature and pressure conditions between the critical conditions, 191 K, 4.69 MPa (−116° F., 680 psi) and atmospheric conditions 106 K, 0.1 MPa (−268° F., 14.7 psi). Ladkany's design consists of a 47 mm (1.85 inch) thick aluminum vessel circumferentially reinforced with 17 mm (0.67 in) thick layers of high strength fiberglass epoxy or 51 mm (2 in) thick layers of pultruded glass polyester overwrap and stiffened against buckling by circumferential frames that are placed at 2.16 m (7.1 ft) intervals. The stiffening frames are also used for structurally supporting and fastening the free-standing vessel during transportation and operation. The metal liners for the hoop-wound pressure vessel are load-sharing and are not bonded to the composite overwrap. Stiffening frames are therefore required for buckling resistance, which adds to the complexity of the design and limits the size of the pressure vessel. Ladkany opts for a welded aluminum pressure vessel for containing the intermediate temperature LNG.
U.S. Pat. No. 5,499,739 (Greist, III et al.) discusses a thermoplastic liner made of a modified nylon 6 or nylon 11 material for use in a pressure vessel to control gas permeation and allow operation at low temperatures, the low end of which is stated to be −40° C. (−40° F.). U.S. Pat. No. 5,658,013 (Bees et al.) discusses a fuel tank for vehicles for holding and dispensing both a liquid and gaseous fuel, and suggests that fully-composite or fiberglass reinforced materials could be used in construction thereof. The liquid fuels discussed in the patent are conventional liquid fuels at ambient temperature and pressure. Both Bees et al. and Mitlitsky et al., previously discussed, propose metal-coated, polymer-based liners that provide further enhancements in performance factors of their tanks/vessels. However, the complexity and hence high cost of the metal deposition process and the liner fabrication process make the tanks/vessels of Bees et al. and Mitlitsky et al. suitable primarily for applications where maximized payload-carrying capacity is the primary objective and, thus, low tank/vessel weight is of very high premium. U.S. Pat. No. 5,695,839 (Yamada et al.) discusses a composite container that has a gas barrier property, wherein the packaging material for constituting such a container is caused to have a laminate structure, and a layer of a material such as an aluminum foil is disposed or interposed in the laminate structure. However, none of the containers discussed in these publications are designed for containing fluids that are at both temperatures less than −40° C. (−40° F.) and high pressures, such as the temperatures and pressures of PLNG.
Conventional liquefied natural gas (“LNG”) is typically transported by sea at temperatures of about −162° C. (−260° F.) and at atmospheric pressure using spherical or close-to-spherical tanks (often called Moss Spheres) made of aluminum or steel capable of cryogenic service. The service pressure for these spherical tanks is too low for PLNG application. Designing very large tanks for the PLNG service pressure using conventional materials presents fabrication challenges due to the unusually large material thicknesses required. Containers for storing and transporting PLNG as described in the PLNG Patent are constructed from ultra-high strength, low alloy steels. However, in spite of the high strength of the steels used in the construction of the PLNG containers described in the PLNG Patent, the weight of a containment system using these containers will be high relative to the cargo and will constrain the ship design through parameters such as draft and stability. Further, these containers will likely be cylindrical in shape and have small diameters, relative to a typical Moss Sphere LNG container, and thus will likely require interconnection with cryogenic-grade materials into a smaller number of containers to simplify loading and unloading. Furthermore, the arrangement of the cylindrical containers will likely affect the geometric design of the ship affecting the ship block coefficient and hence increasing the power requirement, and obstructing the line-of-sight from the engine room. As used herein, the ship block coefficient is defined as V/(L)(B)(T) where V is the volume of fluid displaced by the ship, L is the length between the ship's perpendiculars, B is the ship's beam and T is the ship's draft.
The Non-load-bearing Liner Patent proposes an alternative containment system design based on lightweight, high-performance composite containers with non-load-bearing liners. The reduced weight enhances the ship design by removing weight-related constraints. However, the fabrication complexity of thin-lined composite containers limits the size and geometry of the containers and thereby increases the complexity of piping requirements and impact on geometric design of the ship.
In spite of the aforementioned advances in technology, even those providing systems and methods for producing and storing pressurized liquefied natural gas (PLNG), it would be advantageous to have improved containers and methods for storing and transporting PLNG.
Therefore, an object of this invention is to provide such improved containers and methods. Other objects of this invention will be made apparent by the following description of the invention.